Ignition characteristics and mechanisms of boron in CO₂: Effects of temperature, pressure and CO₂ concentration

The ignition characteristics of amorphous boron (B) particles in carbon dioxide (CO₂) were experimentally investigated using a reflected shock tube over a wide temperature range from 2100 K to 3400 K. The shock tube was equipped with one spectrometer and two monochromators, enabling a comprehensive exploration of the influence of particle size, temperature, pressure and concentration of CO₂ on the ignition delay time (t_ign). Notably, it was found that above 2850 K, t_ign became more dependent on the temperature, which might be attributed to the pyrolysis of CO₂. Meanwhile, t_ign became more sensitive to the pressure with the decrease of temperature. Specifically, the temporal emission spectra of BO₂ from B and pure B₂O₃ were detected at 2600 K in both CO₂ diluted with argon (Ar) and pure Ar atmospheres. Different from the B oxidation in the pure Ar atmosphere, the normalized BO₂ spectral signals and their first-order derivatives indicated two ignition stages of B particles in CO₂. That is, the first ignition happens in the oxide layer, and the subsequent reaction is on the boron surface. These two stages involved three main reactions: (1) pyrolysis of B₂O₃ to BO₂, (2) reaction of B₂O₃ and CO₂, and (3) reaction of B and CO₂. Finally, a theoretical analysis based on valence electron configurations and chemical bonding was performed to explain the bilateral diffusion in stage (1) at the atomic scale.